1. Field of the Invention
The present invention relates to a charged-particle beam instrument and to a method of detecting information from a specimen using a charged-particle beam.
2. Description of Related Art
Charged-particle beam instruments having the configuration of a scanning electron microscope (SEM) have been developed as instruments for inspecting specimens, such as semiconductor wafers. In such an instrument, an electron beam is used as the charged-particle beam. When a specimen is scanned with this electron beam, information emanating from the specimen is detected.
Normally performed inspection of specimens is to detect electrons, such as secondary electrons, emanating from a specimen when it is scanned with an electron beam. Based on the detection, a scanned image of the specimen is obtained. Inspection of the specimen is carried out by checking the scanned image obtained in this way.
The structure of such a charged-particle beam instrument is shown in FIG. 1. The instrument has an electron gun 1 that is a source of a beam of charged particles. The gun 1 emits an electron beam 6 which is accelerated by a given accelerating voltage and directed at a specimen 8. The specimen 8 is a substrate to be inspected, such as a semiconductor wafer. The specimen is positioned within a specimen chamber 15 by a specimen stage (not shown).
The electron beam 6 emitted from the electron gun 1 is made to pass through an aperture mechanism 2 to appropriately limit the beam current at the specimen. The beam 6 passed through the aperture mechanism 2 is passed through a deflection system 3 consisting of two deflectors 3a and 3b and through an objective lens 5. Then, the beam arrives at the specimen 8.
At this time, the deflection system 3 appropriately deflects the beam 6. A scanning unit 11 is connected with the deflection system 3 such that a desired scan signal is supplied to the deflection system 3 from the scanning unit 11. The deflection system 3 is driven based on the scan signal supplied from the scanning unit 11. As a result, the electron beam 6 is deflected.
The objective lens 5 focuses the electron beam 6 onto the surface of the specimen 8. The beam 6 focused by the objective lens 5 is scanned over the specimen 8 by deflecting action of the deflection means 3. An electron optical microscope column 4 is formed by the electron gun 1, aperture mechanism 2, deflection means 3, and objective lens 5.
Electrons 7 to be detected, such as secondary electrons, are produced from the region of the specimen 8 scanned by the electron beam 6. The electrons 7 are detected by an electron detector 9 disposed within the specimen chamber 15. The detector 9 produces an output signal indicative of the result of detection of the electrons 7.
The output signal from the electron detector 9 is amplified by an amplifier 10 and supplied to a display unit 12 consisting of a CRT or the like disposed outside the specimen chamber 15. The display unit 12 is supplied with a scan signal from the scanning unit 11. A scanned image of the specimen 8 based on the output signal from the detector is displayed on the display unit 12 by synchronizing the scan signal with the output signal from the detector.
An accelerating voltage power supply 13 is used to supply an accelerating voltage for accelerating the electron beam 6 emitted from the electron gun 1. The accelerating voltage supplied by the voltage power supply 13 assumes a desired negative potential relative to ground potential and is applied to the electron gun 1.
If necessary, a different negative potential is applied to the specimen 8 from a specimen application voltage power supply 14. Where a negative potential is applied to the specimen 8, the electron beam 6 directed at the specimen 8 is decelerated immediately prior to arrival at the specimen 8. This permits high-resolution imaging of the specimen 8.
Where the specimen 8 is inspected using the charged-particle beam instrument constructed as described above, tilted observation of the specimen surface can be made by tilting the specimen 8 as shown in FIG. 2. In this case, the specimen 8 is tilted by tilting the specimen stage (not shown) carrying the specimen 8 thereon.
In another method being discussed, an auxiliary deflector 16 is mounted between an objective lens 5 and a specimen 8 as shown in FIG. 3. This deflector 16 is energized to tilt an electron beam 6 impinging on the specimen 8.
In a further method being discussed, an electron beam 6 is offset by deflecting action of deflection system 3 as shown in FIG. 4 such that the beam 6 is incident on the main face of the objective lens 5 at an angle. In this case, aberrations, such as coma, are produced in the electron beam 6 because the beam 6 is incident on the objective lens 5 at an angle. Therefore, it has been proposed to mount an auxiliary coil 17 to correct the aberrations.
In one known structure, axisymmetrical electrodes between which an electron beam can pass are disposed in the electron beam passage within the objective lens to perform high-resolution imaging of a tilted specimen (see, for example, Japanese Patent Laid-Open No. H8-185823).
In semiconductor fabrication processes in which semiconductor wafers undergo various processing steps, such as photolithography, many of defects produced in the production steps are caused by problems occurring at the outer peripheries of wafers. Often, such defects are caused by the fact that the outer peripheries of wafers are supported by guide devices and fixing devices to align the semiconductor wafers. Accordingly, it is necessary to make observation of side and rear surfaces of the outer periphery of each wafer as well as tilted observation of the surface of the wafer by SEM, in order to identify the problems caused by defects produced around the outer periphery of the semiconductor wafer.
Where one tries to inspect side and rear surfaces of the outer periphery of a semiconductor wafer that is a specimen by the use of a charged-particle beam instrument having the SEM structure described above, it is necessary to tilt the specimen stage by more than 90°. This complicates the structure of the specimen stage. In addition, there is the problem that the stage is made very bulky.
In the method of deflecting the charged-particle beam by deflection means, the deflection angle of the primary beam relative to the optical axis can be increased up to approximately 15°. However, a function of deflecting the beam even to the rear surface of the specimen is not yet achieved.